The closest beach to Metamorphosis (Sithonia) is Nikiti Beach, 6.5 km away. Within 15 km there are 18 beaches; below are the 12 closest to Metamorphosis, ordered by distance. Photos, map, reviews.
6.5 km from Metamorphosis

7.6 km from Metamorphosis

8.0 km from Metamorphosis

8.6 km from Metamorphosis

9.5 km from Metamorphosis

9.7 km from Metamorphosis

10.3 km from Metamorphosis

10.4 km from Metamorphosis

10.5 km from Metamorphosis

10.5 km from Metamorphosis

10.9 km from Metamorphosis

11.0 km from Metamorphosis

Nikiti Beach, just 6.5 km from Metamorphosis. The rest are listed below, ordered by distance.
Within 15 km of Metamorphosis there are 18 beaches, with 69 across Sithonia in total.
Based on user reviews: Nikiti Beach (★3.8), Schoinia Beach (★3.8), Elaiona Beach (★3.8).
The beaches are spread out around Metamorphosis. A car makes exploring much easier — local buses and taxis are limited outside peak season.
Metamorphosis is on the Sithonia side of Halkidiki, in northern Greece. The nearest airport is Thessaloniki (SKG); from there a car is the most convenient way to reach the Sithonia villages.